Solubility of nano-sized metal oxides evaluated by using in vitro simulated lung and gastrointestinal fluids: implication for health risks

The solubility of nano-sized metal oxides (nZnO, nCuO, nTiO₂, nCeO₂, and nFe₃O₄, 17–42 nm) and some non-nano-mineral powders (ZnO, ZnSiO₃, ZnS, and CuO) were evaluated by using gastrointestinal solubility bioavailability research consortium (SBRC), in vitro gastrointestinal (IVG) method, pulmonary artificial lysosomal fluid (ALF), and Gamble solution method, respectively. It is found that these nano-sized metal oxides aggregated more or less when suspending in the simulated biological fluids analyzed by dynamic light scattering (2 mg L⁻¹) and UV-Vis spectrometry (100 mg L⁻¹). The aggregation and sedimentation of nano-metal oxides in a simulated biofluid are influenced by its surface property and the ingredient of the liquid. The dissolution in fluids may decrease the aggregating radius of a nano-metal oxide. In return, the aggregative effect can influence the solubility of metal elements and result in their weakened bioaccessibility. The suspending stability was consistent in the order of nFe₃O₄ < nCuO < nTiO₂ < nCeO₂ < nZnO in all the simulated biological fluids. Nano-ZnO and nCuO showed higher gastrointestinal and pulmonary bioaccessibility than nFe₃O₄, nTiO₂, and nCeO₂. The further comparisons on the bioaccessibility for nCuO and nZnO with non-nano-powder CuO and ZnO indicated that the aggregating size in suspension could play more important role in influencing the bioaccessibility than single particle size does. The present study reveals that aggregation of all studied nano-sized metal oxides occurred in body physiologic fluids and that nZnO and nCuO were easily dissolved in simulated physiologic fluids, suggesting more potential health risks from nZnO and nCuO’s exposure.